#21 Can't write multiple holding registers

[Mike]

I am connecting to mod-rssim version 8.20 using modbus TCP using modpoll version 3.1 across a private network.
If I enter "modpoll -1 -r1 -t4 192.168.1.3 16", modpoll sends a valid "write multiple registers" with just one register and the value 16.
mod-rssim responds with a normal acknowledgement and updates the register.
However if I enter "modpoll -1 -r1 -t4 192.168.1.3 16 17", modpoll sends a valid "write multiple registers" with two registers and the values 16 and 17.
mod-rssim responds with an "Illegal data value" exception and doesn't update any registers.
Wireshark capture file attached.

[Mike]

wireshark capture file